Field of the Invention
The invention is based on a roll, in particular for use in a machine for producing and/or finishing a fibrous web such as a paper, board or tissue web, including a roll shell having at least some sections formed of a metal material and a coating formed thereon.
Rolls of this type are present in a multiplicity of positions and with a multiplicity of functions in the aforementioned machines. For example, rolls having soft covering layers are suitable for pressing and dewatering the fibrous web, rolls with hard surfaces, in particular including those with a heating device, are primarily used for calendering and drying.
The last-named rolls were earlier often produced from granite and ground with high quality. More recent concepts provided steel rolls, which were likewise polished. Since these rolls all have various disadvantages, such as, for example, the high weight in the case of granite rolls or the susceptibility to corrosion in the case of steel rolls, novel methods for coating roll bodies made of steel or of composite materials have become widespread over time, said methods forming a metallic, ceramic or cermet sprayed layer by means of methods such as HVOF or flame spraying on the roll body. Rolls of this type have been known for a relatively long time and form the current prior art, for example in the case of central press rolls, drying cylinders and guide rolls.
The thermal coating process provides for powder or wires to be melted by means of the input of thermal energy and accelerated kinetically onto the roll core to be coated. The properties and possibilities of the spraying processes are substantially given by the ratio of the kinetic to the thermal energy.
A roll having a coating made of a metal oxide is known, for example from EP 0 870 867 B1. There, a description is given of a roll for a paper machine, board machine or a finishing machine, having a ceramic layer with a thickness of 100 to 2000 μm which is applied to the surface of the roll, the roughness Ra of the outer surface of the roll being 0.2 to 2.0 μm and preferably 0.4 to 1.5 μm. The ceramic layer has 50 to 95% and preferably 55 to 80% of Cr2O3 and 3 to 50% and preferably 20 to 45% of TiO2 and possibly other metal oxides.
The known methods and the metallic, ceramic or cermet coatings that can be produced thereby are afflicted with various disadvantages.
Firstly, as a result of the sole input of energy into the material to be applied and a cooling rate of up to 106 K/s, only a mechanical connection or bonding of the raw material on the roll core occurs. In order to configure this bonding as effectively as possible, careful preparatory surface treatment such as sandblasting and the like is necessary, which is complicated and time-consuming.
Secondly, the coating structure, depending on the material and/or production process, is sealed porous as far as open porosity. The porosity can lead to an increased tendency to corrosion and to adhesion problems. Accordingly, the surface of the sprayed layer must be filled, for example by means of final sealing, with a polymer.
Furthermore, thermal spraying is not an economical or an ecological method, because of the poor application efficiency of the powder with regard to the microns per pass and the percentage of powder remaining on the roll.